The present disclosure relates generally to microfluidic devices and methods for fabricating the microfluidic devices. More particularly, the present disclosure relates to microfluidic devices and methods for fabricating microfluidic devices having encapsulated fluidic tubing and encapsulated electrodes. The microfluidic devices and methods for fabricating the microfluidic devices of the present disclosure provide stronger and more robust microfluidic devices and have stable fluidic interconnects with reduced or no dead volume at the fluidic interconnects. The microfluidic devices and methods for fabricating the microfluidic devices of the present disclosure also allow for coplanar alignment of the fluidic tubing, electrodes and other device features.
Microfluidic device (also referred to herein as microchips) systems are becoming widely used analytical tools for a variety of applications. There are numerous advantages of these systems, one of which is the integration of multiple processes. This can include coupling off-chip processes to the microchip devices as well as integrating multiple processes on-chip. Using microfluidic devices for these types of integration can result in minimal dilution, fast analysis, and improved temporal resolution so that changes in analyte concentration can be monitored in close to real-time. There are numerous examples of integrating multiple processes with microfluidic devices. For example, some processes integrated with microfluidic devices can include coupling conventional capillary electrophoresis (CE) with microfabricated cell traps for single cell analysis, combining capillary liquid chromatography with on-chip CE and electrospray ionization, integrating microdialysis sampling with microchip-based analysis, using multi-layer microfluidic devices for integrating cell culture with analysis, integrating flowing red blood cells, cultured endothelial cells, and electrodes for measuring transendothelial electrical resistance, and the coupling of digital microfluidic devices with mass spectrometry detection.
Integrating off-chip processes to the microchip devices includes fabricating microchip devices with electrodes, fluidic tubing and other features such as, for example, valving techniques, pipettors, discrete injections, plate readers, and off-chip detectors. It has been demonstrated that electrodes can be integrated with microfluidic devices by encapsulating electrodes in epoxy to form an electrode base. The resulting electrode base can be coupled with a polydimethylsiloxane (PDMS) layer to form a microfluidic device. A 3-dimensional arrangement of the electrodes can be created in such a device, and multiple electrode materials can be used to integrate microchip electrophoresis with electrochemical detection.
Another method for making electrodes for microfluidic devices is to create an electrode base via sputtering of metal materials and traditional photolithography. These are typically termed “thin layer electrodes” and are ˜0.1 microns in height. The types of materials that can be made this way are usually limited to just gold or platinum and generally have a very small surface area that limits their analytical performance.
Fluidic tubing can also be integrated with the devices by inserting the fluidic tubing into the epoxy base layer or through a top layer made from soft material such as PDMS to form a fluid interconnect between the fluidic tubing and the microchannel of the microfluidic device. Integration of the fluidic tubing with the microchannel to form the fluid interconnect, however, results in an increased dead volume because of the portion of fluidic tubing that extends or protrudes into the microchannel. The manner with which the fluidic tubing is interfaced with the microchannel can significantly affect integrating off-chip processes with the microchip because of the dead volume created by the fluidic tubing-microchannel interface. For continuous flow processes, the dead volume associated with the fluidic tubing connection may not be crucial. But, in cases where dilution effects are an issue, such as where the analyte concentration rapidly changes as a function of time, when high temporal measurements are desired, or when the sample volume is limited, the fluidic tubing-microchannel interface connection becomes a factor.
One method of coupling fluidic tubing with a microchip involves inserting the tubing into or onto the outer surface of the device, such as through a PDMS layer, and positioning the fluidic tubing outlet into a microchannel of the PDMS layer. However, the amount of dead volume with this type of connection can vary depending upon how far the fluidic tubing is pressed into the device and/or the size of the hole used to interface with the microchannel. Further, because PDMS is a soft material, this type of inserted connection is not very stable. For example, the PDMS material can be damaged during insertion of the fluidic tubing, such as by inadvertent contact with the fluidic tubing, or the fluidic tubing outlet can become inadvertently repositioned within the microchannel, or pulled out of the microchannel entirely. Any such perturbation to the fluidic tubing can also cause leakage or cause the microchip to no longer be sealed, resulting in fluid leaks.
Another method of coupling fluidic tubing that has been used is inserting tubing into the side of the microchip, where the fluidic tubing is butted flush with the microchannel. While this has been shown to result in low dead volume interconnects, fabrication of the device is not trivial. For both methods, there are often issues with the stability of the fluidic tubing-microchannel connection, and complexity of the device increases when multiple fluidic tubing interconnects are needed.
Accordingly, there is a need for microfluidic devices and methods of fabricating microfluidic devices. The microfluidic devices and methods of the present disclosure advantageously include fluidic tubing having stable fluidic interconnects and a reduced or no dead volume. The microfluidic devices and methods of the present disclosure also advantageously allow for alignment of the fluidic tubing, electrodes and other device features along the same plane of the device. Further, base layers can be polished to result in a flat surface that makes bonding with a microchannel layer more robust. Since the alignment can be fixed and the fluidic tubing, electrodes and other device features are encapsulated in the same base layer, connections are straightforward and originate from the same side of the device, which can be critical for high throughput, automation studies.